NetworkTables Communication

NetworkTables is WPILib’s publish-subscribe communication system for sharing data between the robot, driver station, dashboards, and coprocessors in real-time.

What is NetworkTables?

NetworkTables provides:

Automatic Synchronization: Changes propagate to all connected clients
Persistent Data: Values survive program restarts
Topic-Based: Organized by hierarchical names like /SmartDashboard/Speed
Type-Safe: Supports primitives, arrays, and structured data
Low Latency: Optimized for real-time robotics

Getting Started

Getting the Default Instance

#include <networktables/NetworkTableInstance.h>

auto inst = nt::NetworkTableInstance::GetDefault();

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:131

Getting a Table

Tables organize related entries:

#include <networktables/NetworkTable.h>

auto table = inst.GetTable("SmartDashboard");
auto customTable = inst.GetTable("Vision");

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:435

Publishing and Subscribing

Using Topics (NT4 - Recommended)

Topics provide type-safe, efficient communication:

// Publish a double value
auto speedTopic = inst.GetDoubleTopic("/robot/speed");
auto speedPub = speedTopic.Publish();

// In periodic code
void TeleopPeriodic() {
    speedPub.Set(encoder.GetRate());
}

// Subscribe to a value
auto targetTopic = inst.GetDoubleTopic("/vision/target_x");
auto targetSub = targetTopic.Subscribe(0.0);  // Default value

void AutonomousPeriodic() {
    double targetX = targetSub.Get();
    // Use target position...
}

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:199

Supported Topic Types

// Primitives
auto boolTopic = inst.GetBooleanTopic("/enabled");
auto intTopic = inst.GetIntegerTopic("/count");
auto floatTopic = inst.GetFloatTopic("/temperature");
auto doubleTopic = inst.GetDoubleTopic("/velocity");
auto stringTopic = inst.GetStringTopic("/status");

// Arrays
auto doubleArrayTopic = inst.GetDoubleArrayTopic("/positions");
auto stringArrayTopic = inst.GetStringArrayTopic("/labels");
auto boolArrayTopic = inst.GetBooleanArrayTopic("/switches");

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:177

Structured Data

Publish complex types using structs:

#include <frc/geometry/Pose2d.h>

// Publish a Pose2d
auto poseTopic = inst.GetStructTopic<frc::Pose2d>("robot_pose");
auto posePub = poseTopic.Publish();

posePub.Set(frc::Pose2d{1_m, 2_m, 45_deg});

// Subscribe to a Pose2d
auto poseEstimateTopic = inst.GetStructTopic<frc::Pose2d>("vision_pose");
auto poseSub = poseEstimateTopic.Subscribe(frc::Pose2d{});

auto estimatedPose = poseSub.Get();

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:279

Using NetworkTable Entries

Simple Put/Get API

For basic use cases:

auto table = inst.GetTable("SmartDashboard");

// Put values
table->PutNumber("Speed", encoder.GetRate());
table->PutBoolean("AtTarget", controller.AtSetpoint());
table->PutString("Status", "Running");

// Get values
double maxSpeed = table->GetNumber("MaxSpeed", 3.0);  // Default 3.0
bool autoEnabled = table->GetBoolean("AutoEnabled", false);
std::string mode = table->GetString("Mode", "Teleop");

Location: ntcore/src/main/native/include/networktables/NetworkTable.h:361

Array Values

// Put arrays
std::vector<double> positions = {1.0, 2.5, 3.7};
table->PutNumberArray("JointPositions", positions);

std::vector<std::string> names = {"Front", "Back", "Left", "Right"};
table->PutStringArray("SensorNames", names);

// Get arrays
auto defaults = std::vector<double>{0.0, 0.0, 0.0};
auto readings = table->GetNumberArray("Sensors", defaults);

Location: ntcore/src/main/native/include/networktables/NetworkTable.h:490

Listeners and Callbacks

React to value changes:

// Listen to topic changes
auto listener = inst.AddListener(
    speedTopic,
    nt::EventFlags::kValueAll,
    [](const nt::Event& event) {
        if (auto valueData = event.GetValueEventData()) {
            fmt::print("Speed changed to: {}\n", 
                      valueData->value.GetDouble());
        }
    }
);

// Listen to all topics with a prefix
auto prefixListener = inst.AddListener(
    {"/vision"},  // Topic prefix
    nt::EventFlags::kValueAll,
    [](const nt::Event& event) {
        fmt::print("Vision data updated\n");
    }
);

// Remove listener when done
nt::NetworkTableInstance::RemoveListener(listener);

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:515

Server and Client Modes

Robot (Server Mode)

The robot typically runs as a server:

// Start server (usually called automatically by WPILib)
inst.StartServer();

// Or start with custom settings
inst.StartServer(
    "networktables.json",  // Persistence file
    "",                     // Listen on all interfaces  
    1735,                  // NT3 port
    1736                   // NT4 port
);

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:617

Coprocessor (Client Mode)

Vision coprocessors connect as clients:

// Start NT4 client
inst.StartClient4("vision-pi");

// Set server address
inst.SetServerTeam(1234);  // Connect to team 1234's robot

// Or use specific IP
inst.SetServer("10.12.34.2");

// For multiple servers (failover)
inst.SetServer({
    {"10.12.34.2", 1736},
    {"roborio-1234-frc.local", 1736}
});

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:645

Common Patterns

SmartDashboard Integration

#include <frc/smartdashboard/SmartDashboard.h>

// SmartDashboard uses NetworkTables internally
frc::SmartDashboard::PutNumber("Speed", speed);
frc::SmartDashboard::PutBoolean("Limit Switch", limitSwitch.Get());

double kP = frc::SmartDashboard::GetNumber("kP", 1.0);

Tunable Parameters

Adjust PID gains from dashboard:

class TunablePIDController {
public:
    TunablePIDController(std::string_view name) {
        auto table = nt::NetworkTableInstance::GetDefault()
            .GetTable("PID")
            ->GetSubTable(name);
        
        table->PutNumber("kP", 1.0);
        table->PutNumber("kI", 0.0);
        table->PutNumber("kD", 0.0);
        
        kpEntry = table->GetEntry("kP");
        kiEntry = table->GetEntry("kI");
        kdEntry = table->GetEntry("kD");
    }
    
    void UpdateGains() {
        controller.SetP(kpEntry.GetDouble(1.0));
        controller.SetI(kiEntry.GetDouble(0.0));
        controller.SetD(kdEntry.GetDouble(0.0));
    }
    
private:
    frc::PIDController controller{1.0, 0.0, 0.0};
    nt::NetworkTableEntry kpEntry;
    nt::NetworkTableEntry kiEntry;
    nt::NetworkTableEntry kdEntry;
};

Vision Data Exchange

// On vision coprocessor
auto visionTable = inst.GetTable("vision");

void ProcessFrame() {
    auto targets = detectTargets();
    
    if (!targets.empty()) {
        visionTable->PutBoolean("hasTarget", true);
        visionTable->PutNumber("targetX", targets[0].x);
        visionTable->PutNumber("targetY", targets[0].y);
        visionTable->PutNumber("targetArea", targets[0].area);
    } else {
        visionTable->PutBoolean("hasTarget", false);
    }
}

// On robot
void AutonomousPeriodic() {
    auto visionTable = inst.GetTable("vision");
    
    if (visionTable->GetBoolean("hasTarget", false)) {
        double targetX = visionTable->GetNumber("targetX", 0.0);
        double targetY = visionTable->GetNumber("targetY", 0.0);
        
        // Aim at target
        aimAtTarget(targetX, targetY);
    }
}

Persistent Values

Values that survive robot reboots:

auto table = inst.GetTable("Config");

// Make entry persistent
table->SetPersistent("OffsetAngle");
table->PutNumber("OffsetAngle", 15.0);

// Value will be saved and restored on next boot

Location: ntcore/src/main/native/include/networktables/NetworkTable.h:336

Connection Management

Check Connection Status

if (inst.IsConnected()) {
    // Robot is connected to driver station or client
}

// Get connection info
auto connections = inst.GetConnections();
for (const auto& conn : connections) {
    fmt::print("Connected to: {}\n", conn.remote_id);
}

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:747

Flush Updates

// Force immediate network update (normally automatic)
inst.Flush();

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:730

Best Practices

Use Topics Over Entries: NT4 topics are more efficient than legacy entries
Batch Updates: Group related updates to minimize network traffic
Minimize Publish Rate: Don’t publish faster than necessary (20-50Hz is often sufficient)
Use Structured Types: Prefer structs over multiple individual values
Name Hierarchically: Use clear paths like /subsystem/component/value
Persist Configuration: Use persistent entries for tunable parameters
Handle Disconnections: Always provide default values for Get operations

Debugging

View with OutlineViewer

WPILib includes OutlineViewer for browsing NetworkTables:

Shows all topics and values
Edit values in real-time
Monitor connection status

Logging

// Enable NetworkTables logging
auto logger = inst.AddLogger(
    nt::LogLevel::kLogInfo,
    nt::LogLevel::kLogDebug,
    [](const nt::Event& event) {
        if (auto logData = event.GetLogMessage()) {
            fmt::print("NT: {}\n", logData->message);
        }
    }
);

Location: ntcore/src/main/native/include/networktables/NetworkTableInstance.h:837

Performance Considerations

Topic Count: Minimize number of topics for better performance Update Rate: Limit how often you publish:

// Bad: Updates every cycle (50Hz)
void RobotPeriodic() {
    topic.Set(getValue());
}

// Good: Updates at 20Hz
void RobotPeriodic() {
    static int counter = 0;
    if (++counter % 3 == 0) {  // Every 3rd cycle = 20Hz
        topic.Set(getValue());
    }
}

Array Size: Keep arrays reasonably sized (< 1KB typical) String Length: Avoid very long strings in high-frequency updates

Getting Started

Core Concepts

Programming Guide

Simulation

Advanced Topics

Tools

NetworkTables Communication

What is NetworkTables?

Getting Started

Getting the Default Instance

Getting a Table

Publishing and Subscribing

Using Topics (NT4 - Recommended)

Supported Topic Types

Structured Data

Using NetworkTable Entries

Simple Put/Get API

Array Values

Listeners and Callbacks

Server and Client Modes

Robot (Server Mode)

Coprocessor (Client Mode)

Common Patterns

SmartDashboard Integration

Tunable Parameters

Vision Data Exchange

Persistent Values

Connection Management

Check Connection Status

Flush Updates

Best Practices

Debugging

View with OutlineViewer

Logging

Performance Considerations

Getting Started

Core Concepts

Programming Guide

Simulation

Advanced Topics

Tools

Documentation Index

​What is NetworkTables?

​Getting Started

​Getting the Default Instance

​Getting a Table

​Publishing and Subscribing

​Using Topics (NT4 - Recommended)

​Supported Topic Types

​Structured Data

​Using NetworkTable Entries

​Simple Put/Get API

​Array Values

​Listeners and Callbacks

​Server and Client Modes

​Robot (Server Mode)

​Coprocessor (Client Mode)

​Common Patterns

​SmartDashboard Integration

​Tunable Parameters

​Vision Data Exchange

​Persistent Values

​Connection Management

​Check Connection Status

​Flush Updates

​Best Practices

​Debugging

​View with OutlineViewer

​Logging

​Performance Considerations

What is NetworkTables?

Getting Started

Getting the Default Instance

Getting a Table

Publishing and Subscribing

Using Topics (NT4 - Recommended)

Supported Topic Types

Structured Data

Using NetworkTable Entries

Simple Put/Get API

Array Values

Listeners and Callbacks

Server and Client Modes

Robot (Server Mode)

Coprocessor (Client Mode)

Common Patterns

SmartDashboard Integration

Tunable Parameters

Vision Data Exchange

Persistent Values

Connection Management

Check Connection Status

Flush Updates

Best Practices

Debugging

View with OutlineViewer

Logging

Performance Considerations